Method of manufacturing powder molding and powder feeder

ABSTRACT

A material powder is packed in a cavity of a mold with high and uniform density to manufacture a powder molding a material powder in a shoe box 12 is fed into a cavity 6 formed in a mold 1 by moving the shoe box 12 over the cavity 6 while oscillating the shoe box until the density of the powder in the cavity increases to at least 1.1 times the apparent density. The powder is then compression-molded.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing a powder moldingusing a mold for compression molding, and a powder feeder for use in themanufacture of such a power molding.

2. State of the Prior Art

FIG. 6 illustrates a conventional method of manufacturing a powdermolding. In this method, a powder molding is manufactured by moving apowder feed shoe box 2 containing powder to a position over a cavity 21formed in a mold 20 to drop the powder in the box 22 into the cavity 21,backing the shoe box 22, and compressing the powder in the cavity 21 bylowering an upper punch 23.

A powder molding is usually sintered subsequently. If the density of thepowder molding is uneven, the dimension of the powder molding tends tobe uneven when sintered, so that the sintered product tends to be unevenin dimension, too.

In this conventional method, powder is dropped by gravity into thecavity 21, so that the powder tends to form a bridge in the cavity. Thisleads to uneven density of the powder.

The powder in the shoe box 22 spontaneously drops into the cavity 21while moving the shoe box 22 to over the cavity. Then, by backing theshoe box 22, any portion of the powder protruding from the surface ofthe mold 20 is scraped off by the edge of the shoe box 22, so that thetop of the powder in the cavity is leveled out. This causes unevennessin the density of the powder.

In order to make the density of the powder packed in the cavity of themold uniform, trials were made to vibrate the mold after filling powderin the cavity. But by vibrating the mold, the mold tends to displace ormay be worn. If the mold moves, it may be broken by interfering with theupper punch.

Unexamined Japanese Patent Publication 5-69195 discloses a method offeeding powder in which a high-frequency AC current is supplied througha coil surrounding the mold to microscopically oscillate a materialpowder containing magnetic substances by producing an eddy current inthe powder.

But in this method, it is impossible to use a powder other than magneticpowders. Also, an extra space has to be provided around the mold tomount the coil. This leads to reduced rigidity of the die set. Moreover,a magnetic field produced by the coil tends to unnecessarily magnetizethe mold and the powder molding. In order to oscillate the powder with asufficient strength, a high voltage is required. This pushes up theproduction cost.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method of manufacturing apowder molding which makes it possible to pack a material powder in acavity of a mold with high and uniform density, and a powder feeder foruse in this method.

According to this invention, there is provided a method of manufacturinga powder molding comprising the steps of feeding a material powder intoa cavity formed in a mold for compression molding through a bottomopening formed in a shoe box while oscillating the material powder whileit is in the shoe box until the density of the powder in the cavityincreases to at least 1.1 times the apparent density, and compressingthe powder in the cavity.

The powder in the shoe box should preferably be oscillated at anoscillating frequency of 10-200 Hz and an oscillating pressure of 1-50kg.

The powder feeder for use in the manufacture of a powder moldingcomprises a shoe box mounted to move toward and away from a cavityformed in a mold for compression molding, and an oscillator mounted onthe shoe box for oscillating the shoe box.

In order to effectively oscillate the powder in the shoe box, aplurality of top- and bottom-open cells are preferably provided in theshoe box at its lower portion.

By feeding powder from the shoe box into the cavity of the mold, whilevibrating the shoe box, the vibration of the shoe box is transmittedthrough the powder in the shoe box to the powder in the cavity, so thatthe density of the powder in the cavity increases.

If the density of the powder in the cavity is at a valve at least 1.1times the apparent density, the value is close to the upper limit, sothat the density of the powder is made sufficiently uniform. If thisvalue is less than 1.1 times the apparent density, a variation indensity, i.e. a difference between the maximum and minimum densities ofthe powder, will increase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a device for use in the manufacturingmethod according to this invention.

FIG. 2 is a plan view in cross-section of the shoe box of the same.

FIG. 3 is a vertical sectional front view of FIG. 2.

FIG. 4 is a graph showing the relationship between the oscillatingfrequency applied to the material powder and the variation in density ofthe article formed by the method of the present invention.

FIG. 5 is a graph showing the relationship between the oscillatingpressure applied to the material powder and the variation in density ofthe article formed by the method of the present invention.

FIG. 6 is a schematic view of a conventional device for use in themanufacture of powder moldings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-3 show an embodiment of this invention.

FIG. 1 shows a device for manufacturing a powder molding. It comprises amold 1 and a powder feeder 10 for feeding powder into the mold 1.

The mold 1 comprises an elevatable die holder 2, a die 3 supported onthe die holder 2, a cylindrical lower punch 4 inserted in the die 3, acore rod 5 inserted in the lower punch 4, and an upper punch 7 adaptedto be lowered into a cavity 6 defined over the lower punch 4.

The powder feeder 10 comprises a hopper 11 filled with a materialpowder, and a powder feed shoe box 12 slidable on the die holder 2 andconnected to the hopper 11 through a flexible hose 13.

The shoe box 12 is moved toward and away from the cavity 6 of themold 1. It carries an oscillator 14.

The oscillator 14 may be pneumatic, activated by air pressure, orelectric. But the pneumatic type is preferable because it produces lessnoise and is less expensive.

FIGS. 2 and 3 show the detailed structure of the powder feed shoe box12. It has an opening at its bottom. The lower part of the interior ofthe shoe box 12 is partitioned by a plurality of partitioning plates 15aand 15b that intersect with each other at a right angle into a pluralityof cells 16 whose tops and bottoms are open.

The oscillation of the shoe box 12 induced by the oscillator 14 travelsthrough the partitioning plates 15a, 15b to the powder in the box. Ifthe cells 16 are too large, it is impossible to effectively oscillatethe powder. If too small, the cells may be clogged with powder due tothe friction between the plates 15a, 15b and the powder.

Thus, each cell 16 should be sized so that the distance from its centerto the inner surface of the partitioning plates 15a, 15b will be between0.5 mm and 20 mm.

The cells 16 may have a square section as shown, or may be cylindrical,or may be of any other desired shape.

When the shoe box 12 is moved to right over the cavity 6 of the mold 1,the powder in the hopper 11 flows through the hose into the shoe box 12.Then, it flows through the cells 16 and the bottom opening of the box12, and drops into the cavity 6 by gravity.

While the powder is being fed into the cavity, the shoe box 12 isoscillated by activating the oscillator 14.

The oscillation of the shoe box 12 is transmitted to the powder in thecells 16 and then to the powder in the cavity 6.

By oscillating the powder while feeding it into the cavity, it ispossible to increase the density of the powder in the cavity. Byincreasing its density to 110% or more of the apparent density, itapproaches its limit, so that the density of the powder in the cavity ismade uniform.

If the oscillator 14 is oscillated at a frequency lower than 10 Hz, itwill take a long time to feed powder uniformly into the cavity. Ifhigher than 200 Hz, the amplitude of oscillation would decrease to suchan extent that the powder can hardly follow oscillation. This makes itimpossible to fill the cavity with powder with sufficient density.

For the foregoing reason, the oscillator 14 should be oscillated atfrequencies between 10 Hz and 200 Hz.

If the oscillating pressure is less than 1 kg, it is impossible tosufficiently oscillate the powder in the shoe box 12, so that it willtake a long time until the density of the powder in the cavity 6 becomessufficiently uniform. If larger than 50 kg, the amplitude of vibrationwill increase to such an extent that the powder moves so violently thatit cannot be fed into the cavity by gravity. Also, the amount of weardue to the oscillation of the shoe box 12 increases with the amplitudeof oscillation. Thus, too large an amplitude of vibration can shortenthe life of the shoe box 12, posing an economic problem. The vibrationpressure should therefore be about 1-50 kg.

If the oscillating time per cycle exceeds 10 seconds, the powder moldingtime will increase. This increases the cost of mass-production. Thus,the oscillating time per cycle should not exceed 10 seconds.

After feeding powder into the cavity in the above-described manner, theshoe box 12 is backed, and then the upper punch 7 is lowered to compressthe powder in the cavity 6 for molding.

Now description is made of experiments of methods of manufacturingpowder moldings using the powder feeder according to this invention.

Experiment 1

Pure iron powder having an average grain diameter of 100 μm was put intothe hopper 11 shown in FIG. 1, so as to feed it into the shoe box 12through the hose 13.

In this state, the shoe box 12 was moved over the cavity 6 of the mold1, and simultaneously the oscillator 14 was activated to feed the powderinto the cavity 6 while oscillating the powder.

The shoe box 12 used had a 110-by-110 mm regular square section with nopartitioning plates 15a, 15b provided inside.

The mold 1 used had a ring-shaped cavity 6 having an outer diameter of40 mm and an inner diameter of 27 mm.

The oscillator 14 used was a pneumatic type. It was operated at anoscillating frequency of 30Hz and a vibrating pressure of 10 kg for 5seconds per cycle.

After filling the cavity with powder, the upper punch 7 was lowered tocompress the powder in the cavity at a pressure of 6 tons/cm². Then, thecompression-molded article was taken out by lowering the die holder 2.The amount of powder packed and variation in density were measured. Theresults are shown in Table 1.

As a comparative example, we also prepared a compression-molded articlewhich was formed by feeding powder into the cavity 6 while notoscillating the shoe box 12, and compressing it. The results ofmeasurements of the comparative example are also listed in Table 1.

The "Variation in molding density" was obtained by diametricallydividing each article into eight segments, measuring the densities forthe respective segments, and subtracting the minimum one of the eightdensity values from the maximum one.

                  TABLE 1                                                         ______________________________________                                                    Amount of powder                                                                         Variation in                                                       packed     molding density                                        ______________________________________                                        Molding 1     43.1 g       0.11 g/cm.sup.3                                    Comparative article                                                                         39.0 g       0.20 g/cm.sup.3                                    ______________________________________                                    

It is apparent from Table 1 that the article formed according to themethod of the invention was more than 10% higher in the amount of powderpacked than the comparative article and that this fact means lowervariation in density.

Experiment 2

Powder moldings were formed in the same manner as in Experiment 1, usingshoe boxes 12 having partitioning plates 15a and 15b arranged atintervals of 40 mm, 30 mm, 10 mm and 1 mm. We measured the amount ofpowder packed and variation in molding density for Articles 2, 3, 4, and5 which were formed using the shoe boxes 12 having their partitioningplates arranged at intervals of 40 mm, 30 mm, 10 mm and 1 mm,respectively. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Amount of     Variation in                                                                              Distance from center of                             powder packed molding density                                                                           opening to inner wall of                            g             g/cm.sup.3  pertitioning plate (mm)                             ______________________________________                                        Molding 2                                                                            43.3       0.10        20                                              Molding 3                                                                            44.5       0.06        15                                              Molding 4                                                                            45.1       0.04        5                                               Molding 5                                                                            39.5       0.25        0.5                                             ______________________________________                                    

As is apparent from Table 2, the measurement results for Article 2differ little from those for Article 1. This is because the partitioningplates 15a, 15b were arranged too far apart from each other.

In contrast, Articles 3 and 4 achieved marked improvements both in theamount of powder packed and variation in density. Article 5, which wasformed with the Partitioning plates 15a, 15b arranged too close to eachother, was low in the amount of powder packed and high in variation indensity.

Experiment 3

Powder moldings were formed using a shoe box having its partitioningplates 15a, 15b arranged at intervals of 10 mm in the same way as inExperiment 1 except that the oscillating frequency and oscillatingpressure were changed. We measured the variation in molding density foreach powder molding obtained.

As shown in FIG. 4, the variation in density was too large atoscillating frequencies of less than 10 Hz or more than 200 Hz, and thesmallest at frequencies near 30 Hz.

As will be apparent from FIG. 5, where the oscillating pressure was lessthan 1 kg, it was impossible to oscillate the powder sufficiently, sothat the variation in density was large. Also, where the vibrationpressure was higher than 50 kg, the variation in density increased dueto too large, an oscillating amplitude.

Industrial Application

According to this invention, a material powder is fed into the cavity ofthe mold while oscillating the shoe box. The powder is thus packed withuniform and high density. The article formed by compressing the powderin the cavity shows excellent properties.

Since only the shoe box is oscillated, the mold is less likely to bedamaged. Also, with this arrangement, it is possible to use practicallyany kind of material powder.

By providing a plurality of mutually partitioned cells in the shoe box,vibration of the shoe box can be effectively transmitted to the powderin the shoe box, so that it is possible to pack powder uniformly in thecavity to a high level of density in a short time.

We claim:
 1. A method of manufacturing a powder molding, comprising thesteps of:feeding a material powder from a shoe box through a bottom ofthe shoe box into a cavity formed in a mold while oscillating thematerial powder at an oscillation frequency of 10-200 Hz and at anoscillation pressure of 1-50 kg such that the density of the powder inthe cavity becomes at least 1.1 times the apparent density; andcompressing the powder in the cavity.
 2. The method of claim 1, whereinsaid step of feeding comprises moving the shoe box over the cavity, andwherein said step of compressing comprises lowering a punch into saidcavity after moving said shoe box away from said cavity.
 3. The methodof claim 1, wherein the shoe box has an oscillator mounted thereto.
 4. Apowder feeder for use in manufacturing a powder mold, comprising:a moldpart having a cavity for compression molding; and a shoe box mounted soas to be movable toward and away from said cavity, said shoe box havinga lower portion provided with a plurality of cells having open tops andbottoms, said cells each comprising a space defined in said lowerportion by a plurality of vertical partitioning plates; and anoscillator mounted on said shoe box.
 5. The powder feeder of claim 4,wherein said shoe box is connected to a powder hopper by a flexiblehose.
 6. The powder feeder of claim 4, wherein said mold part comprisesa punch adapted to be lowered into said cavity.
 7. The powder feeder ofclaim 4, wherein said mold part comprises an upper surface, said cavityopening onto said upper surface, and wherein said shoe box is movablealong said upper surface between a position over said cavity and aposition in which said cavity is uncovered by said shoe box.